Encapsulant composition and electronic package utilizing same

ABSTRACT

A composition for use in making an encapsulant usable in the encapsulation of a semiconductor chip assembled to a substrate with C4 solder interconnections therebetween, which in turn may form part of an electronic package. The composition comprises a resin, a flexibilizing agent and a filler material.

TECHNICAL FIELD

This invention relates to a composition such as may be used forencapsulating a semiconductor chip on a substrate as part of anelectronic package.

BACKGROUND OF THE INVENTION

Controlled collapse chip connection (C4) or flip-chip technology hasbeen successfully used for over twenty years for interconnecting highI/O (input/output) count and area array solder bumps on silicon chips tobase ceramic chip carriers, for example alumina carriers. The solderbump, typically a lead/tin (Pb/Sn) alloy such as 95 Pb/5 Sn alloyprovides the means of chip attachment to the ceramic chip carrier forsubsequent usage and testing. This is described in U.S. Pat. Nos.3,401,126 and 3,429,040 to Miller and assigned to the assignee of thepresent application. Typically, malleable pads of metallic solder areformed on semiconductor chip contact sites and solder joinable sites areformed on conductors on the chip carrier. The chip carrier solderjoinable sites are surrounded by non-solderable barriers so that whenthe solder on the semiconductor chip contact sites melts, to form ajoint, surface tension of the molten solder prevents collapse of thejoints and thus holds the semiconductor chip suspended above the chipcarrier.

Usually the circuit semiconductor chips are mounted on supportingsubstrates made of materials with coefficients of thermal expansion thatdiffer from the coefficient of thermal expansion of the material of thesemiconductor chip, i.e. silicon. Normally the semiconductor chips areformed of monocrystalline silicon with a coefficient of thermalexpansion of about 2.5 parts per million (ppm.)/degree Celsius (° C.)and the substrate is formed of a ceramic material, typically aluminawith a coefficient of thermal expansion of about 5.8 ppm./° C. Inoperation, the active and passive elements of each integratedsemiconductor chip inevitably generate heat resulting in temperaturefluctuations in both the chips and the supporting substrate since theheat is conducted through the solder joints. The chips and the substratethus expand and contract in different amounts with temperaturefluctuations, due to the different coefficients of thermal expansion.This imposes stresses on the relatively rigid semiconductor chip solderjoints.

The stress on the semiconductor chip solder joints during operation isdirectly proportional to (1) the magnitude of the temperaturefluctuations, (2) the distance of an individual joint from the neutralor central point (DNP), and (3) the difference in the coefficients ofthermal expansion of the material of the semiconductor chip and thesubstrate, and inversely proportional to the height of the solder joint,that is the spacing between the chip and the support substrate. Theseriousness of the situation is further compounded by the fact that asthe solder joints become smaller in diameter in order to accommodate theneed for greater I/O density, the overall height decreases. U.S. Pat.No. 4,604,644 to Beckham, et al. and assigned to the assignee of thepresent application, describes a structure for electrically joining asemiconductor chip to a support substrate that has a plurality of solderconnections where each solder connection is joined to a solder wettablepad on the chip and a corresponding solder wettable pad on the supportsubstrate. Dielectric organic material is disposed between theperipheral area of the chip and the facing area of the substrate, thematerial surrounds at least one outer row and column of solderconnections but leaves the solder connections in the central area of thedevice free of dielectric organic material. The preferred materialdisclosed in U.S. Pat. No. 4,604,644 is obtained from a polyimide resinavailable commercially and sold under the product name AI-10 by BP-AmocoChemical Corporation, Chicago, Ill. AI-10 is formed by reacting adiamine such as p,p′diaminodiphenylmethane with trimellitic anhydride oracylchloride of trimellitic anhydride. The polymer is further reactedwith gamma.-amino propyl triethoxy silane or β-(3,4-epoxy cyclohexyl)ethyltrimethoxy silane. The coating material is described in IBM TDBSeptember 1970 P. 825.

More recently, U.S. Pat. No. 5,668,904, and assigned to the assignee ofthe present application, describes a method of increasing the fatiguelife of solder interconnections between a semiconductor chip and asupporting substrate. The method includes attaching the semiconductorchip to the substrate by a plurality of solder connections that extendfrom the supporting substrate to electrodes on the semiconductor chip toform a gap between the supporting substrate and the semiconductor chip.The gap is filled with a composition consisting of a cycloaliphaticpolyepoxide and/or cyanate ester or prepolymers, and fillers such asaluminum nitride or aluminum oxide. The composition is then cured.

Although the above techniques have been quite successful in improvingfatigue life of solder interconnections between a semiconductor chip anda supporting substrate, there still remains room for improvement inextending the fatigue life. An improved encapsulant composition formaking an encapsulant has been developed to further improve fatigue lifeof solder interconnections between a semiconductor chip and a supportingsubstrate. It is believed that such a composition and the resultantelectronic package will constitute a significant advancement in the art.

OBJECTS AND SUMMARY OF THE INVENTION

Accordingly, it is the object of this invention to provide a new andunique composition which in turn may be used as an encapsulant in anelectronic package.

Another object of this invention is to provide a method of making suchan encapsulant composition.

Yet another object of this invention is to provide an electronic packageincluding a substrate, a semiconductor chip positioned on the substrate,and an encapsulant composition positioned on the substrate and on aportion of the semiconductor chip, the composition having a resinmaterial, a flexibilizing agent, and a filler as part thereof. Theencapsulant composition will improve the fatigue life of solderinterconnections between the semiconductor chip and the substrate.

The invention is adaptable to mass production and improves theoperational field life of product made with the invention.

According to one aspect of the invention, there is provided anencapsulant composition comprising a resin material, a flexibilizingagent, and a filler material.

According to another aspect of the invention, there is provided a methodof making an encapsulant composition, the method comprising the steps ofproviding a first quantity of resin material, adding to the firstquantity of resin material a second quantity of flexibilizing agent,adding to the first quantity of resin material a third quantity offiller material, and blending the resin material.

According to yet another aspect of the invention, there is provided anelectronic package comprising a substrate having an upper surface, asemiconductor chip mounted on a portion of the upper surface of thesubstrate and electrically coupled to the substrate, the semiconductorchip having a bottom surface and at least one edge surface beingsubstantially perpendicular to the bottom surface, and a materialpositioned on at least the portion of the upper surface of the substrateand against at least a portion of the at least one edge surface of thesemiconductor chip, the material having an encapsulant composition whichincludes a resin material, a flexiblizing agent, and a filler material.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a sectional view in elevation of one embodiment of theelectronic package of the present invention, illustrating the substratehaving an upper surface, the semiconductor chip mounted on a portion ofthe upper surface of the substrate and electrically coupled to thesubstrate.

BEST MODE FOR CARRYING OUT THE INVENTION

In accordance with the teachings of this invention, there is provided anew composition for use as an encapsulant in the manufacture of a chipcarrier usable as part of an electronic package. In one embodiment, thisencapsulant composition comprises a resin material, a flexibilizingagent, and a filler material. It has been discovered that when thiscomposition is utilized in the assembly of a semiconductor chip onto acarrier to make a chip carrier, it results in a chip carrier packagehaving improved operational field life. Specifically, during acceleratedthermal cycling from between about −65° C. to about 125° C. hair linecracks may form along the corners of encapsulated semiconductor chips orin a plane between the encapsulant and the chip passivation layer. Suchcracks, once initated, can grow during thermal cycling and result incatastrophic failure of the solder interconnections between thesemiconductor chip and carrier to which it is assembled, decreasing theoperational field life of the electronic package. The currentencapsulant composition provides improved operational field life bysubstantially preventing the formation of such cracks.

The resin material of this invention is selected from the groupconsisting of epoxy and cyanate ester resins. The resin materialcomprises about 20% to about 55% by weight of the composition. Examplesof epoxies that can be used in this invention are selected fromnon-glycidyl ether epoxides containing more than one 1,2-epoxy group permolecule. These are generally prepared by epoxidizing unsaturatedaromatic hydrocarbon compounds, such as cyclo-olefins, using hydrogenperoxide or peracids such as peracetic acid and perbenzoic acid. Theorganic peracids are generally prepared by reacting hydrogen peroxidewith either carboxylic acids, acid chlorides, or ketones, to give thecompound R—COOOH. These materials are well known, and reference may bemade to Brydson, J., Plastic Materials, 1966, page 471, for theirsynthesis and description.

Such non-glycidyl ether cycloaliphatic epoxides are characterized byhaving a ring structure wherein the epoxide group may be part of thering or attached to the ring structure. These epoxides may also containester linkages. The ester linkages are generally not near the epoxidegroup and are relatively inert to reactions.

Examples of useful non-glycidyl ether cycloaliphatic epoxides include3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate,vinylcyclohexane dioxide;3,4-epoxy-6-methylcyclohexylmethyl-3,4-epoxycyclohexane carboxylate anddicyclopentadiene dioxide. A distinguishing feature of many of thecycloalipahtic epoxides is the location of the epoxy group(s) on a ringstructure rather than on an aliphatic side chain. Generally, thecycoaliphatic epoxides particularly useful in this invention will havethe formula:

where S stands for a saturated ring structure, R is selected from thegroup consisting of CHOCH₂, O(CH₂)_(n)CHOCH₂, and OC(CH₃)₂CHOCH₂radicals, where n is 1 to 5. R′ is selected from the group consisting ofhydrogen, methyl, ethyl, propyl, butyl, and benzyl radicals and R″ isselected from the group consisting of CH₂OOC and CH₂ OOC(CH₂)₄COOradicals.

These cycloaliphatic epoxy resins may be characterized by reference totheir epoxy equivalent weight, which is defined as the weight of epoxidein grams which contains one gram equivalent of epoxy. Suitablecycloaliphatic epoxy resins have a preferred epoxy equivalent weight ofabout 50 to about 250 grams per equivalent of epoxy. They will generallyhave a viscosity between about 5 to about 900 centapoise (cps) at 25° C.

Examples of cycloaliphatic epoxides are suggested in U.S. Pat. Nos.3,207,357; 2,890,194; 2,890,197; and 4,294,746, the disclosures of whichare hereby incorporated herein by reference. A discussion of variouscycloaliphatic epoxides can be found in the publication entitled“Cycloaliphatic Epoxide Systems,” Union Carbide Corporation, 1970, thedisclosure of which is hereby incorporated herein by reference. Mixturesof cycloaliphatic epoxides can be employed when desired. Cycloaliphaticepoxies are usually low viscosity liquids at room temperature and caneither be used alone or as reactive diluents in blends with thesemi-solid glycidyl ether epoxies. These materials include3′,4′-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate, which isavailable from Union Carbide Corporation, Danbury, Conn. under theproduct name ERL-4221 and available from Vantico Inc. 5121 San FernandoRoad, Los Angeles, Calif., under the product name ARALDITE CY-179;diglycidylester of hexahydrophthalic anhydride available from VanticoInc. under the product name CY-184; bis (3,4-epoxycyclohexylmethyl)adipate, available from Union Carbide Corporation under the product nameERL-4299; the isomeric mixture of bis (2,3-epoxycyclopentyl) ether,available from Union Carbide Corporation under the trademark nameERL-4205; ERL-4205 reacted with ethylene glycol or blended with abisphenol A based diglycidyl ether, which were once available from UnionCarbide Corporation under the product names ERLB-4617 and ERL-2258,respectively.

Other suitable epoxy resins which can be incorporated in the presentinvention include, for example, those represented by the followingformulas, I–IV:

wherein each A is independently a divalent hydrocarbyl group having from1 to about 9, and preferably from 1 to 4, carbon atoms, —O—, —SO₂—, or—CO—; each A′ is independently a divalent hydrocarbyl group having fromabout 1 to about 9, and preferably from 1 to 4 carbon atoms; Q is ahydrocarbyl group having from about 1 to about 10 carbon atoms; Q′ ishydrogen or an alkyl group having from about 1 to about 4 carbon atoms;each X is independently hydrogen, bromine, chlorine, or a hydrocarbylgroup having from about 1 to about 9 and preferably from 1 to 4 carbonatoms; m has an average value of 0 to about 12, and preferably from 0.03to 9, and most preferably from 0.03 to 3; m′ has a value from about0.011 to about 10, and preferably from 0.05 to 6; n has a value of 0 or1; and n′ has an average value from 0 to about 10, preferably from about0.1 to about 3.

Suitable epoxy resins include, for example, the diglycidyl ethers ofresorcinol, catechol, hydroquinone, biphenol, bisphenol A,tetrabromobisphenol A, phenolaldehyde novolac resins, alkyl substitutedphenol-aldehyde resins, bisphenol F, tetramethylbiphenol,tetramethyltetrabromophenol, tetramethyltetrabromophenol,tetrachlorobisphenol A, combinations thereof, and the like.

The epoxy resin monomers or prepolymers may be virtually any of avariety of commercially available materials. The glycidyl ethers ofvarious phenolic compounds are particularly important. These include theglycidyl ethers of bisphenol A.

These resins are widely available from a number of manufacturers such asShell Chemical Company, DOW Chemical Company, and Vantico in a varietyof molecular weights and viscosities. Examples include: D.E.R. 332,D.E.R. 330, D.E.R. 331, D.E.R. 383, D.E.R. 661, TACTIX 123, TACTIX 138,and TACTIX 177 all products of DOW Chemical Company, 2030 Dow Center,Midland, Mich.; EPON 825, EPON 826, and EPON 828 all products of ShellChemical Company, 910-T Louisiana St., Houston, Tex.; and, ARALDITE GY6008, ARALDITE GY 6010, and ARALDITE GY2600 all product s of VanticoInc. Additionally, flame retardant epoxy resins can be used includingfluorinated or brominated bisphenol type epoxy resins under the productnames D.E.R. 542 and D.E.R. 566-A80 from DOW Chemical Company. Anotherimportant class of glycidyl ethers are those of phenolic novolac andcresol novolac resins. These materials are also widely available from anumber of manufacturers in a variety of molecular weights andviscosities. Examples include Epon 862 and Epon 155, which are productsof the Shell Chemical Company; D.E.R. 354, D.E.N. 431, D.E.N. 438, andD.E.N. 439 which are products of DOW Chemical Company; and ARALDITE PY306, ARALDITE EPN 1139, ARALDITE EPN 1138, ARALDITE GY 281, ARALDITE GY285, ARALDITE GY 302-2, ARALDITE LY 9703, ARALDITE XD 4955, and ARALDITEECN 9511, which are product s of Vantico Inc.

A similar epoxy that may also be used is SU-8, a product of ShellChemical Corporation. Several other polyfunctional glycidyl ethers areof significant importance for high performance applications, i.e. heavymechanical loads under conditions of high temperature and harshenvironment. The materials include: the tetraglycidyl ether of tetrakis(4-hydroxyphenyl)ethane, which is commercially available from the ShellChemical Company under the product name EPON 1031 and from Vantico, Inc.under the product name ARALDITE MT 0163. The diglycidyl ether of9,9-bis(4-hydroxyphenyl) fluorene is commercially available from ShellChemical Corporation under the Product name EPON HPT 1079. Glycidylethers of the condensation product of dicyclopentadiene and phenol areavailable from DOW Chemical Company under the product name TACTIX 556.

The triglycidyl ether of tris(hydroxyphenyl) methane, which is availablefrom DOW Chemical Company under the product names TACTIX 742 or XD9053.EPON 1031, EPON HPT 1079, TACTIX 556, TACTIX 742 and XD9053, are eitherhigh viscosity liquids or solids at room temperature. Therefore it isadvantageous to blend these materials with a low viscosity bisphenol Aor bisphenol F based diglycidyl ether or reactive diluents. Theresulting blends are less viscous at ambient temperatures and are moreeasily processed. Some heating may be required for adequate flow, butthe temperatures needed are not high enough to cause thermal curing ofthe epoxy group. Specific blends were found to have a good overallcombination of low viscosity in the uncured state and high glasstransition temperature, flexural strength, and modulus when cured. It isparticularly advantageous to blend a high performance semi-solid epoxysuch as TACTIX 556, TACTIX 742 or EPON HPT 1079, with a low viscositybisphenol A or bisphenol F based glycidyl ether epoxy such as EPON 862,TACTIX 123, or a reactive diluent. EPON 862 is a product of ShellChemical Corporation.

The cyanate esters that can be employed pursuant to the presentinvention have two or more —O—CN groups and are curable through acyclotrimerization reaction. The cyanate esters can be monomeric or lesspreferably polymeric, including oligomers and can be represented bythose materials containing the following group:

where A represents independently a single bond, —C(CH₃)(H)—, —SO₂—, —O—,—C(CF₂)₂—, —CH₂OCH₂—, —S—, —C(═O)—. —O—C(═O)—O—, —S(═O)—, —O—P(═O)—O—,—O—P(═O)(═O)—O—, divalent alkylene radicals such as —CH₂— and —C(CH₃)₂—,or divalent alkylene radicals interrupted by heteroatoms in the chainsuch as O, S, and N.

Each R is independently selected from the group of hydrogen, alkylcontaining 1 to 9 carbon atoms. Each n independently is an integer of 0to 4.

Other cyanates useful in the method, composition, and structure of theinvention can be prepared by well known methods, for example, byreacting the corresponding polyvalent phenol with a halogenated cyanate,as described in U.S. Pat. Nos. 3,553,244; 3,740,348; and 3,755,402.

The phenol reactant can be any aromatic compound containing one or morereactive hydroxyl groups. The phenolic reactant is preferably a di- ortri-polyhydroxy compound of the formula:

in which each a and b is independently 0, 1, 2, or 3, and at least one ais not 0; n is within the range of 0 to about 8, preferably 0 to 3; eachR is independently selected from non-interfering alkyl, aryl, alkaryl,heteroatomic, heterocyclic, carbonyloxy, carboxy, and the like ringsubstituents, such as hydrogen, C₁₋₆ alkyl, C₁₋₆ allyl, C₁₋₆ alkoxy,halogen, maleimide, propargyl ether, glycidyl ether, and the like; and Ais a polyvalent linking moiety which can be, for example, aromatic,aliphatic, cycloaliphatic, polycyclic, and heteroatomic. Examples oflinking moiety A include —O—, —SO₂—, —CO—, —OCOO—, —S—, —C₁₋₁₂—,dicyclopentadienyl, aralkyl, aryl, cycloaliphatic, and a direct bond.

Specific cyanate esters that can be employed in the present inventionare available and well-known and include those discussed in U.S. Pat.Nos. 4,195,132; 3,681,292; 4,740,584; 4,745,215; 4,477,629; and4,546,131; European patent application EP0147548/82; and German Offen.2611796, disclosures of which are incorporated herein by reference. Apreferred polyfunctional cyanate ester is bisphenol AD disyanate(4.4′-ethylidenebisphenoldicyanate) available from Vantico Inc. underthe trade designation AROCY L-10, hexafluoro bisphenol A dicyanate(Arocy-40S), and bisphenol M dicyanate (RTX366) both commerciallyavailable from Vantico Inc.

The compositions of this invention utilize a flexibilizer whichcomprises about 1.0% to about 5% by weight of the composition. Thepurpose of the flexibilizer is to impart desirable mechanical propertiesto the cured composition, such as flexibility and thermal shockresistance especially when such compositions can experience temperatureexcursions below −40° C. Flexibilizing agents including options such asa thermoplastic, hydroxy-containing thermoplastic oligomer, epoxy orother organic functional reactive-containing thermoplastic oligomer,reactive flexibilizer, rubber, elastomer, epoxy functionalizedflexibilizers, engineering thermoplastics, and amine orhydroxy-terminated thermoplastic oligomers or mixtures thereof could beused in the present invention to provide cured compositions having highglass transition temperatures, good mechanical properties, and goodtoughness at low temperatures.

The epoxy resin monomer flexibilizing agents described above may also beadvantageously modified by mixing with various additives. Such additivesinclude polyols such as ethylene glycol, propylene glycol, 1,3-butyleneglycol, 1,4-butylene glycol, and other glycols. Aromatic diphenols andpolyphenolic compounds may also be used to modify the epoxy resin. Otherreactive diluents, which contain vinyl, acrylate, or methacrylate may beemployed to change reactivity, glass transition temperature, ormechanical properties. In addition, reactive diluents based onmonofunctional or polyfunctional glycidyl ethers may also be used toreduce the viscosity or modify the resin systems.

High molecular weight engineering thermoplastics are particularlyeffective at increasing the flexibility of the thermally or radiativelycured epoxy or epoxy-triazine mixtures utilized as binding matrices.Polysulfones which are available under the product names UDEL and RADELfrom BP-Amoco Chemicals, Chicago, Ill., can be dissolved in the epoxyresin composition to form a viscous homogeneous mixture. Similar resultscan be obtained with a polyetherimide available under the product nameULTEM from General Electric Plastics, Pittsfield, Mass. It is notnecessary for the thermoplastic to be miscible with the triazine ortriazine epoxy resin composition. The addition of Nylon 12 and Nylon6/12 particles, under the product names ELF ATOCHEM ORGASOL 2001 andORGASOL 3501, respectively, both available from Atofina Chemicals, Inc.,Philadelphia, Pa., can result in improved fracture toughness even thoughthese materials are insoluble in the epoxy resin monomer mixture.Similar results may also be obtained using insoluble polyimideparticles, available under the product name IMITEC X-902 from ImitecInc., 1990 Maxon Road, Schenectady, N.Y. Other thermoplastics such aspolyamideimides, poly(arylene ethers), polyesters, polyarylates,polycarbonates, polyurethanes, and others are potentially useful asflexibilizers in the present invention, examples of which can be foundin “Engineering Plastics, “D. C. Clagett, Encyclopedia of PolymerScience and Engineering”, John Wiley and Sons, final edition.

Engineering thermoplastics are typically endcapped with nonreactivefunctional groups. It may also be advantageous for the flexibilizingagent to be a low molecular weight segment or oligomer of a previouslydescribed engineering thermoplastic, which contains functional groupsthat are capable of reaction with the cyanate or epoxy-cyanate resinduring thermally induced polymerization. Accordingly, thermoplasticmaterials that have been modified to contain a thermoplastic oligomerbackbone and to have more reactive end groups are particularly useful asflexibilizers. For this purpose hydroxy-terminated polysulfone oligomersbased on the UDEL P-1700 polymer backbone can be synthesized at variousmolecular weights. UDEL P-1700 is one of the UDEL products previouslydescribed. These materials can be more easily blended with the resinmonomer mixture and the resulting compositions are less viscous thanthose having the same percentage of high molecular weight polymer ofsimilar backbone, but with different end groups. These materials arealso found to be very effective in increasing fracture toughness.Oligomers with other backbones can also be used, particularly those ofpoly(arylene ethers), polyarylates, and polyesters. Conceivably, theoligomer backbone could be that of any of the previously referencedthermoplastics. Reactive end groups are those which react with thecyanate-epoxy resin during thermal polymerization. These groups includehydroxy, epoxy, and carboxylate groups. Flexible molecules which containtwo or more epoxy groups represent a class of material which can alsouseful as flexibilizers for the present invention. These compoundstypically contain long aliphatic groups which act to reduce crosslinkdensity in the cured epoxy resin. In addition to increasing the fracturetoughness of the cured resin, the addition of low viscosityflexibilizers can also significantly reduce the overall viscosity of theuncured resin/flexibilizer mixture. Useful flexibilizers include but arenot limited to: 1,4-butane-diol diglycidyl ethers (product name SHELLHELOXY MODIFIER 67), neopentlyglycol diglycidyl ether (product nameSHELL HELOXY MODIFIER 68), cyclohexane dimethanol diglycidyl ether(product name SHELL HELOXY MODIFIER 107), trimethylol ethane triglycidylethers (product name SHELL HELOXY MODIFIER 44), dibromoneopentylglycolglycidyl ethers (product name SHELL HELOXY MODIFIER 56), propoxylatedglycerol polyglycidyl ether (product name SHELL HELOXY MODIFIER 84),polypropylene glycol glycidyl ether (product name SHELL HELOXY MODIFIER32), polyglycidyl ether of castor oil (product name SHELL HELOXYMODIFIER 505), diner acid diglycidyl esters (product name SHELL HELOXYMODIFIER 71), resorcinol diglycidyl ether (product name SHELL HELOXYMODIFIER 69). These HELOXY MODIFIERS are available from Shell ChemicalCompany. Other examples of useful flexibiliziers include epoxidizedpropylene glycol dioleates (product name ELF ATOCHEM VIKOFLEX 5075),epoxy esters (product name ELF ATOCHEM VIKOFLEX 4050), 1,2-tetradecaneoxides (product name ELF ATOCHEM VIKOFLEX 14), internally epoxidized1,3-butadiene homopolymers (product name ELF ATOCHEM POLY BD 600 andPOLY BD 605). These ATOCHEM flexibilizers are available from AtofinaChemicals Inc. Further examples of flexibilizers useful in thisinvention are diglycidyl ether, glycidyl glycidate,bis(2,3-epoxy-2-methylpropyl)ether, and polyglycoldiepoxides, availableunder the product names DER 732 and DER 736 from DOW Chemical Company.Flexible molecules which contain two or more hydroxy groups are alsouseful as flexibilizers for this invention. These flexible polyolcompounds also contain long aliphatic groups. Useful polyols includeE-caprolacetone triol available under the product names TONE 0301, 0305,0310 from Union Carbide Corp, Danbury, Conn.

Elastomers or rubbers may also be used as flexibilizers. Examples ofthese materials include, but are not limited to, the copolymers ofstyrene, butadiene, and ethylene or styrene, butylene, and ethylene,butadiene, styrene copolymers, copolymers of butadiene and styrene,butyl rubber, neoprene rubber, and poly (siloxanes). Functionalizedversions of these materials such as carboxyl terminated poly(n-butylacrylate) rubber are particularly useful. Epoxy resin monomersmay be reacted with these materials to form an epoxy terminatedelastomer which is useful as a flexibilizer. Maleic anhydride terminatedKRATON rubber, available under the product name FG 1901X from ShellChemical Corporation, and epoxy functionalized liquid KRATON rubbers,available under product names EXP-206 and EXP-207 also from ShellChemical Corp., are especially useful as flexibilizers. Other productsalso include prereacted epoxy resins with butadiene-acrylate copolymers,available as the MRU-283 product series from 3M Corp, St. Paul, Minn.

It may also advantageous to blend the various types of flexibilizers inorder to adjust the overall viscosity of the uncured resin/flexibilizercomposition. A flexibilizer may be added to a mixture of a thermoplasticor thermoplastic oligomer dissolved in an epoxy resin monomer. Overallviscosity may be reduced and toughness may be improved compared withusing a flexibilizer alone. The thermoplastic may separate from thecured epoxy-cyanate resin to form a two phase morphology while theflexibilizer provides long flexible groups to connect crosslink sites inthe network.

The molecular weight of the flexibilizers can range between about 400and about 20,000, more preferably between 500 and 5000. Fluoridizedrubbers or polysiloxanes that are provided with a terminally functionalgroup and hydroxylated or carboxylated EPDM (ethylenepropylene/ethylidene norbornene rubbers can also be used asflexibilizers in the present invention.

The compositions employed pursuant to the present invention also includea filler comprising from about 45 percent to about 75 percent by weightof the composition. Thermally conductive and electrically insulatingfillers can be used for improving the thermal heat transfer from thesemiconductor chip to the surroundings. Such fillers include AluminumOxide, 92% Alumina, 96% Alumina, Aluminum Nitride, Silicon Nitride,Silicon Carbide, Beryllium Oxide, Boron Nitride and Diamond powdereither high pressure or Plasma CVD. The filler material can also be azirconate such as zirconium tungstate having a negative coefficient ofthermal expansion property. These fillers can be used in concentrationsequivalent to fused silica and by blending these into suitable lowviscosity thermosetting resins thermally conductive C4 encapsulatingmedia could be realized. Fused silica is well known in the art and willnot be described here. The filler particles can have substantiallyspherical or spheroidal shapes and have diameters of less than about 41microns, preferably about 0.7 to about 31 microns. This is necessary sothat the composition will readily flow in the gap between the chip andsubstrate carrier. The gap is normally about 25 to about 160 microns.

A portion of each of the filler particles can include a layer of acoupling agent positioned thereon to improve adhesion and moistureresistance. Coupling agents, such as c-aminopropyltriethoxy silane,available under the product name A1100, b-(3,4-epoxycyclohexyl)ethyltrimethoxy silane, available under the product name A186, orc-glycidyl-propyltrimethoxy silane, available under the product nameZ6040 all from Dow Chemical Company can be used in this invention. Anamount of coupling agent which is about 0.25% by weight of filler hasbeen found to be satisfactory. The amount can be determined by weightloss of filler treated with coupler after burning. The thickness of thecoupling agent should be more than about a few monolayers.

In addition to the binder and filler, the compositions can also includea catalyst to promote the polymerization of the epoxy and/or cyanateester.

Suitable catalysts for the epoxy include amines such as the imidazoles,tertiary amine benzyldimethylamine, 1,3-tetramethyl butane diamine,tris(dimethylaminomethyl) phenol, pyridine, and triethylenediamine, andacidic catalysts, such as stannous octoate.

Suitable catalysts for the cyanate ester include Lewis acids, such asaluminum chloride, boron trifluoride, ferric chloride, titaniumchloride, and zinc chloride; salts of weak acids, such as sodiumacetate, sodium cyanide, sodium cyanate, potassium thiocyanate, sodiumbicarbonate, and sodium boronate. Preferred catalysts are metalcarboxylates and metal chelates, such as cobalt, manganese, iron, zinc,and copper acetylacetonate or octoates and naphthenates. The amount ofcatalyst, when used, can vary, and generally will be about 0.005 toabout 5 weight percent, preferably about 0.05 to about 0.5 weightpercent based on total solid binder (resin weight excluding the fillercontent) weight.

Surfactants in amounts of about 0.5% to about 3% by weight of thecomposition and preferably about 1.2% to about 11.6% by weight of thecomposition can be used to facilitate mixing the filler with the epoxy.Suitable surfactants include silanes and non-ionic type surface activeagents, available as polyethylene glycol-p-tert-octylphenyl ether underthe product name Triton X-100 from Rohm and Hass Co., Philadelphia, Pa.Surfactants are generally prepared by the reaction of octylphenol ornonylphenol with ethylene oxide.

The compositions of the present invention may also include an organicdye in amounts less than about 0.2% by weight of the total compositionto provide contrast. Suitable dyes are nigrosine and Orasol blue GNwhich is a tradename of Ciba Specialty Chemicals, Pigments Division,205-T S. James St., Newport, Del. The preferred compositions employedpursuant to the present invention are substantially free (e.g. less than0.2% by weight) if not completely free from non-reactive organicsolvents. Compositions employed pursuant to the present invention haveviscosity at about 25° C. (Brookfield cone & plate Spindle 51, 20 RPM orequivalent) of about 750 cps to about 50,000 cps and preferably about3,000 cps to about 20,000 cps. The as mixed compositions are stable forat least about 12 hours and can be subsequently cured at temperatures ofless than about 200° C., and preferably about 130° C. to about 180° C.,in about 2 to about 6 hours and preferably about 4 to about 5 hours. Thecured compositions have coefficients of thermal expansion of about 25 toabout 40 ppm/° C., glass transition temperatures of greater than about130° C., and preferably about 140° to about 190° C. The curedcompositions have Shore D hardness of greater than about 85 andpreferably greater than about 90.

In accordance with the teachings of this invention, there is alsoprovided an electronic package 10 which includes a substrate 2 having anupper surface 4, a semiconductor chip 6 mounted on a portion of theupper surface of the substrate and electrically coupled to thesubstrate, the semiconductor chip having a bottom surface 8 and at leastone edge surface 12 being substantially perpendicular to the bottomsurface, and a material 14 positioned on at least the portion of theupper surface of the substrate and against at least a portion of the atleast one edge surface of the semiconductor chip, the material having anencapsulant composition, as described in detail above, which includes aresin material, a flexiblizing agent and a filler material.

The substrate of the package can include an organic material, eitherrigid or flexible, including conventional FR-4 epoxy and laminates basedon high temperature resins, such as high temperature epoxies,polyimides, cyanates (triazines), fluoropolymers, benzocyclobutenes,polyphenylenesulfide, polysulfones, polyetherimides, polyetherketones,polyphenylquinoxalines, polybenzoxazoles, polyphenyl benzobisthiazolesand dicyclopentadiene based epoxies, cyanates or other functionalities.The substrate can also include halide free resins. The substrate of thepackage can include a reinforcing material selected from the groupconsisting of organic woven fibers, organic non-woven fibers, inorganicfibers, and inorganic non-woven fibers.

Fluoropolymers include perfluoroalkylenes, as polytetrafluorethylene,copolymers of tetrafluoroethylene and hexaf luoropropylene, copolymersof tetrafluoroethylene and perfluoro-2,2-dimethyl-1,3-dioxide,polytrifluorochloroethylene, copolymers of tetrafluoroethylene witholefins as ethylene, copolymers of trifluorochloromethane with olefinsas ethylene, and polymers of perfluoroalkyl vinyl ether. Somecommercially available fluoropolymers include polytetrafluoroethylene,tetrafluoroethylenne-perfluoroalkoxy, tetrafluoroethylene-ethylene,chlorotrifluoroethylene-ethylene, chlorotrifluoroethylene, andtetrafluoroethylene-perfluoro-2,2-dimethyl-1,3 dioxide.

Commercially available fluorocarbon polymers reinforced with fiber glassparticulates useful in this invention are available from the RogersCorporation, Rogers, Conn. under the trade designations RO2800 andRO2500.

The polymers that can be used as substrates in accordance with thepresent invention may also include unmodified polyimides, as well asmodified polymides such as polyester imides, polyamide-imide-esters,polyamide-imides, polysiloxane-imides as well as other mixed polyimides.Such are well known in the prior art and need not be described ingreater detail.

The substrate can also comprise an inorganic, or compositeorganic/inorganic material. Substrates having thin film redistributionlayers may also be used. Inorganic substrates that can be used in thisinvention comprise ceramic material. The preferred ceramic substratesinclude silicon oxides and silicates, such as aluminum silicate andaluminum oxides and can include a layer of glass material therein. Highheat transfer substrates, such as aluminum nitride substrates, may alsobe used.

In accordance with yet another embodiment of this invention, a methodfor making the encapsulant composition defined above is hereby provided.The method comprises the steps of providing a first quantity of resinmaterial as defined hereinabove, adding to the first quantity of resinmaterial a second quantity of the above-defined flexibilizing agent,adding to the first quantity of resin material a third quantity offiller material of the type defined above, and then blending the resinmaterial under vacuum. Vacuum is used at a pressure of about 5millimeters of mercury. The method can include homogenizing theflexibilizing agent in the first quantity of resin material by reactingthe resin material and the flexibilizing agent together at a temperatureof greater than about 100 degrees ° C.

The encapsulant composition can be applied to the gap between asemiconductor chip assembled to a substrate by C4 interconnections bydispensing the encapsulant composition through nozzles under pressure ofabout 15 pounds per square inch to about 90 pounds per square inch andtemperatures of about 25 degrees ° C. to about 45 degrees ° C. Theencapsulant composition can completely cover the C4 interconnections,the surface of the substrate on which the semiconductor is assembled,and at least a portion of the edges of the device forming a fillet asillustrated in the drawing. It may be desirable to pregell theencapsulant composition by heating for about 15 to about 60 minutes,typically about 30 minutes at about 75° C. to about 100° C. and cure theencapsulant after application.

The encapsulant composition can then be substantially cured by heatingto about 130° C. to about 200° C. and preferably about 130° C. to about180° C. for about 2 hours to about 6 hours and preferably about 2 toabout 4 hours.

While there have been shown and described what are presently consideredthe preferred embodiments of the invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the scope of the invention as defined bythe appended claims.

1. An encapsulant composition comprising: a resin material selected fromthe group consisting of epoxy and cyanate ester resins; from about 1.0%by weight to about 5% by weight of the composition of a flexibilizingagent comprising a flexibilizer containing functional groups capable ofreaction with the epoxy or cyanate ester resin during thermally inducedcuring, and a thermoplastic other than the flexibilizer, wherein thethermoplastic is separated from the cured epoxy or cyanate ester resin;and a filler material comprising substantially spherical or spheroidalparticles, each particle having a diameter of less than about 41microns, wherein the composition has a higher fracture toughness, alower viscosity, and increased thermal shock resistance at a temperatureexcursion below −40° C., or combinations thereof than the compositionwould have if the flexibilizing agent were not present in thecomposition, wherein the thermoplastic comprises a poly(arylene)ether,and wherein the flexibilizer comprisesbis(2,3-epoxy-2-methylpropyl)ether.
 2. The composition of claim 1,wherein a portion of each of said spherical or spheroidal particlesincludes a layer of coupling agent positioned thereon, and wherein thecoupling agent comprises about 0.25% by weight of the filler material.3. An electronic package comprising: a substrate having an uppersurface; a semiconductor chip mounted on a portion of said upper surfaceof said substrate and electrically coupled to said substrate, saidsemiconductor chip having a bottom surface and at least one edge surfacebeing substantially perpendicular to said bottom surface; and a materialpositioned on at least said portion of said upper surface of saidsubstrate and against at least a portion of said at least one edgesurface of said semiconductor chip, said material being the encapsulantcomposition of claim
 1. 4. A method of making an encapsulantcomposition, the method comprising the steps of: providing a firstquantity of resin material selected from the group consisting of epoxyand cyanate ester resins; adding to said first quantity of resinmaterial a second quantity of flexibilizing agent by homogenizing saidflexibilizing agent in said first quantity of resin material by reactingsaid resin material and said flexibilizing agent together at atemperature of greater than about 100 degrees Celsius, from about 1.0%by weight to about 5% by weight of the composition of said flexibilizingagent comprising a flexibilizer containing functional groups capable ofreaction with the epoxy or cyanate ester resin during thermally inducedcuring, and a thermoplastic other than the flexibilizer, wherein thethermoplastic is separated from the cured epoxy or cyanate ester resin;adding to said first quantity of resin material a third quantity offiller material comprising substantially spherical or spheroidalparticles, each particle having a diameter of less than about 41microns; and blending said resin material, wherein after said blendingsaid flexibilizing agent comprises 1 percent to about 5 percent byweight of said composition, wherein immediately after the steps ofadding the flexibilizing agent and adding the filler material have beenperformed the composition is uncured and the composition is stable for atime period at least 12 hours, wherein the composition has a higherfracture toughness, a lower viscosity, and increased thermal shockresistance at a temperature excursion below −40° C., or combinationsthereof than the composition would have if the flexibilizing agent werenot present in the composition, wherein the thermoplastic comprises apoly(arvlene) ether, and wherein the flexibilizer comprisesbis(2,3-epoxv-2-methylpropyl)ether.
 5. The method of claim 4, whereinsaid step of blending is performed under vacuum.
 6. The method of claim5, wherein said step of blending is performed under vacuum at a pressureof about 5 millimeters of mercury.
 7. The method of claim 4, wherein aportion of each of said spherical or spheroidal particles includes alayer of coupling agent positioned thereon, and wherein the couplingagent comprises about 0.25% by weight of the filler material.
 8. Thecomposition of claim 1, wherein the resin material consists of epoxyresins, wherein the composition further comprises a surfactantcomprising that facilitates mixing the filler with the epoxy resins,wherein the surfactant comprises non-ionic type surface active agents,and wherein the surfactant comprises between about 0.5% and about 3% byweight of the composition.
 9. The composition of claim 1, wherein thefiller material has a negative coefficient of thermal expansion.
 10. Thecomposition of claim 1, wherein the composition further comprises anorganic dye comprising less than about 0.2% by weight of thecomposition.
 11. The composition of claim 1, wherein the compositionfurther comprises non-reactive organic solvents comprising less thanabout 0.2% by weight of the composition.
 12. The composition of claim 1,wherein the composition is cured and has a coefficient of thermalexpansion between about 25 and about 40 ppm/° C., a glass transitiontemperature between about 140° and about 190° C., and a Shore D hardnessgreater than about
 90. 13. The method of claim 4, wherein the resinmaterial consists of epoxy resins, wherein the composition furthercomprises a surfactant comprising that facilitates mixing the fillerwith the epoxy resins, wherein the surfactant comprises non-ionic typesurface active agents, and wherein the surfactant comprises betweenabout 0.5% and about 3% by weight of the composition.
 14. The method ofclaim 4, wherein the filler material has a negative coefficient ofthermal expansion.
 15. The method of claim 4, wherein the compositionfurther comprises an organic dye comprising less than about 0.2% byweight of the composition.
 16. The method of claim 4, wherein thecomposition further comprises non-reactive organic solvents comprisingless than about 0.2% by weight of the composition.
 17. The method ofclaim 4, wherein after the substantially curing step, the compositionhas a coefficient of thermal expansion between about 25 and about 40ppm/° C., a glass transition temperature between about 140° and about190° C., and a Shore D hardness greater than about
 90. 18. Thecomposition of claim 1, wherein the resin material consists of epoxyresins, and wherein the epoxy resins comprise3′,4′-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate.
 19. Thecomposition of claim 1, wherein the filler material comprises zirconiumtungstate.
 20. The composition of claim 2, wherein the coupling agentcomprises b-(3,4-epoxycyclohexyl)ethyltrimethoxy silane.
 21. Thecomposition of claim 8, wherein the surfactant comprises polyethyleneglycol-p-tert-octylphenyl ether.
 22. The composition of claim 10,wherein the organic dye comprises nigrosine.
 23. The method of claim 4,wherein the resin material consists of epoxy resins, and wherein theepoxy resins comprise 3′,4′-epoxycyclohexylmethyl3,4-epoxycyclohexanecarboxylate.
 24. The method of claim 4, wherein thefiller material comprises zirconium tungstate.
 25. The method of claim7, wherein the coupling agent comprisesb-(3,4-epoxycyclohexyl)ethyltrimethoxy silane.
 26. The method of claim13, wherein the surfactant comprises polyethyleneglycol-p-tert-octylphenyl ether.
 27. The method of claim 15, wherein theorganic dye comprises nigrosine.